Ruthenium, which is one type of rare metal materials, is used as chemical catalysts and also used as electronic industrial materials for electrodes, resistors and the like, or plating materials. Recently, magnetism of ruthenium has been utilized for hard discs, and thus demand for ruthenium has been remarkably increased. Because of balance between the production amount and demand, cost of ruthenium has been raised. Therefore, it is now desired to collect ruthenium compounds at a high collection ratio from aqueous solutions, waste water and the like containing ruthenium or the like, and to recycle ruthenium. Many methods have been proposed for separating and collecting ruthenium or ruthenium compounds. A general method is an oxidation and distillation method, by which ruthenium tetroxide is generated by use of an oxidizer such as halogen gas, sodium hypochlorite, sodium bromate, ozone or the like and is collected by distillation (see, for example, Patent Documents 1 through 4). However, the oxidation and distillation method has problems that the generated ruthenium tetroxide is unstable and explodes by autolysis, and is also easily corroded (oxidized) and thus is limited in the materials to be used therewith.
Hydroxyapatite, which is one type of calcium phosphate, is known as a compound having a cation exchange capability and adsorbability and being capable of immobilizing ruthenium or the like (see, for example, Non-patent Document 1). A method using hydroxyapatite to put ruthenium or the like into contact with a heavy metal material in an aqueous solution containing phosphoric acid ions and having a pH value of 6 or higher, and thus generating a sparingly soluble product on a surface of a phosphate mineral to remove heavy metal material ions has been proposed (Patent Document 5).
Calcium phosphate compounds are not sufficiently resistant against acids and the adsorbent itself is dissolved under an acidic condition. Therefore, the calcium phosphate compounds are generally handled under a neutral or alkaline condition in which the calcium phosphate compounds can be kept solid. However, when a calcium phosphate compound in a solid state is used as an adsorbent, there is a problem that ions in crystals are not sufficiently released and thus the adsorbability of the calcium phosphate compound cannot be sufficiently utilized.
Other methods for separating and collecting ruthenium or ruthenium compounds include the following: a method by which ruthenium or a ruthenium compound, after being subjected to some pre-process or supplied with a drug, is put into contact with an ion exchange resin, a chelate resin, a resin having no ion exchange group such as polyethylene or the like, activated carbon or the like and thus is separated (see, for example, Patent Documents 6 through 11); a method of generating a precipitate by an amino compound and a heteropoly acid (Patent Document 12); and a method by which ruthenium or a ruthenium compound is mixed with an acid or an alkali, then melted or exudated by use of an oxidizer and thus separated (see, for example, Patent Documents 13 through 16). However, these methods have been developed mainly for the purpose of smelting, or refining and separating, ruthenium or ruthenium compounds from ores, not from a low concentration aqueous solution of ruthenium. Therefore, these methods have problems that, for example, the process is complicated. There are other problems that, for example, when a salt, a lower alcohol or an organic acid which inhibits an oxidation-reduction operation or an ionization operation is contained, the collection ratio is decreased.